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Preparation of Ultrafine YBa2Cu3O7-x Superconductor Powders by the Poly(vinyl alcohol)-Assisted Sol-Gel Method Yang-Kook Sun Daeduk R&D Center, Samsung Heavy Industries Company, Ltd., P.O. Box 43, Daeduk Science Town, Daejeon 305-600, Korea
In-Hwan Oh* Division of Chemical Engineering, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea
The ultrafine high-Tc YBa2Cu3O7-x superconductor powders were prepared by the sol-gel method using poly(vinyl alcohol) (PVA) as a protecting agent of metal ions. The conditions of the sol formation from the aqueous solution of metal nitrates containing PVA were determined, and the thermal decomposition process of the gel precursor was examined to determine the crystallization temperature. It was found that a nearly pure phase of polycrystalline YBa2Cu3O7-x could be obtained by calcining the gel precursor above 900 °C for 2 h in air. Polycrystalline YBa2Cu3O7-x powders prepared at 920 °C consisted of very uniformly-sized ultrafine particulates with an average particle size of about 25 nm and showed a good superconducting property with Tc around 94 K. Introduction Since it has been disclosed that the YBa2Cu3O7-x superconductor showed a transition temperature of 90 K, there have been many reports on the synthesis, structure, and properties of this new material. This new material, however, has many difficulties in forming them into fibers, wires, and other components. It is well-known that the superplastic deformation can enhance the ease of fabrication of brittle oxide materials. In order to achieve superplastic deformation, it is requisite to synthesize oxides with a small grain size and a relatively constant stress at high temperatures (Carry and Mocellin, 1983; Venkatochasi and Rai, 1986). Since powder synthesis methods play a crucial role on the properties of the final material, many synthesis methods have been proposed. Superconducting oxides are usually prepared by grinding and calcining of oxides and carbonates. This solid-state reaction method, however, has several disadvantages: inhomogeneity, nonuniformity, larger particle size, high impurity content, lack of reproducibility, high porosity of the sintered material, necessity of repeated processing, higher sintering temperature, and longer sintering time. Thereby, several solution methods have been developed to overcome these problems, which include coprecipitation of carbonates or oxalates (Chen et al., 1987; Wang et al., 1991), sol-gel methods (Barboux et al., 1988; Chu and Dunn, 1987; Fransaer et al., 1989; Nozue et al., 1991; Sun and Lee, 1993; Villa et al., 1989), and freeze-drying (Johnson et al., 1987). Among these, the sol-gel method can produce highly reactive homogeneous powders. This simple technique has many advantages: highly homogeneous mixing, good stoichiometric control, and production of active submicron-size particles in a relatively shorter processing time at lower temperatures. Also, ceramic fibers and thin films can be produced from the sol-derived gel precursor with such shapes by adjusting pH, temperature, aging time, and viscosity of the solution. * To whom correspondence should be addressed. FAX: +822-958-5199.
S0888-5885(95)00527-6 CCC: $12.00
In general, the fine oxide particles can be fabricated by the sol-gel method. However, there are several methods according to the types of chelating agents. The most general methods are the amorphous citrate method (Chu and Dunn, 1987; Sun and Lee, 1993; Villa et al., 1989) and the Pechini method (Pechini, 1967). The mechanism of the sol formation for the amorphous citrate method, where hydroxycarboxylic acid such as citric acid is used as a chelating agent, is that COOanions, which are formed from the dissociation of the citric acid in water, react with cations to form sols through ionic bonding. In the Pechini method, a mixture of hydroxycarboxylic acid (citric acid) and hydroxyl alcohol (ethylene glycol) is used, in which chelated citric acid plays the role of distributing the cations atomistically throughout the polymeric structure. Sometimes, polymeric hydroxycarboxylic acid such as poly(acrylic acid) has been substituted for citric acid (Lessing, 1989). On the other hand, an alternative sol-gel method where only hydroxyl alcohol was used without the aid of hydroxycarboxylic acid at all has been proposed in the synthesis of nanocrystalline composite oxides La1-xSrxFe1-yCoyO3 with the perovskite structure using poly(ethylene glycol) (Li et al., 1993) and in the lowtemperature preparation of fine particles of mixed oxide systems such as NiFe2O4 using poly(vinyl alcohol) (Saha et al., 1995). In this hydroxyl alcohol-assisted sol-gel method, hydroxyl alcohol just dissolves in water and wraps the cations to form sols, instead of being dissociated in water. Therefore, the sol formation mechanism is essentially different from the sol-gel processes described in the previous paragraph. In this study, the high-Tc YBa2Cu3O7-x superconductor powders with very uniform and ultrafine particulates were prepared by the sol-gel method using poly(vinyl alcohol) (PVA) as a protecting agent of metal ions. The conditions of the sol formation from the aqueous solution of metal nitrates containing PVA were determined, and the thermal decomposition process of the gel precursor was examined to determine the crystallization temperature. The fabrication conditions and the physical properties © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4297 Table 1. Appearance of the Dried Materials Derived from the Solutions at Various Molar Ratios of PVA to the Total Metal Ions molar ratio of PVA to the total metal ions
appearance
3 5 7 10 15 20
transparent sol + 70% precipitate transparent sol + 30% precipitate transparent sol + 10% precipitate transparent sol transparent sol transparent sol
Results and Discussion
Figure 1. Flowsheet of the preparation procedure for polycrystalline YBa2Cu3O7-x powders by the PVA-assisted sol-gel method.
of the YBa2Cu3O7-x superconductor powders obtained were then investigated. Experimental Section YBa2Cu3O7-x superconductor powders were synthesized according to the procedure as shown in Figure 1. A stoichiometric amount of each nitrate salt with the cationic ratio of Y:Ba:Cu ) 1:2:3 was dissolved in distilled water and mixed well with an aqueos solution of PVA. This solution was evaporated at 70-80 °C for 1-2 days until a blue transparent sol was obtained. As the evaporation of water proceeded, the sol turned into a viscous blue gel precursor, which was then decomposed at 500 °C for 2 h in air to eliminate organic contents. The decomposed precursor powders were ground and then calcined at 800-920 °C for 2 h in air to obtain YBa2Cu3O7-x powders. For the preparation of the gel precursor with a different PVA/total metal ion ratio, the same procedure was repeated, with the molar ratio of PVA to the total metal ions being varied as 3, 5, 7, 10, 15, and 20. The thermal decomposition behavior of the gel precursor was examined by means of thermogravimetry (TG) and differential thermal analysis (DTA). Powder X-ray diffraction analysis using Cu KR radiation was used to identify the crystalline phase of the gel and gelderived materials which were calcined at temperatures of 500, 800, 900, and 920 °C for 2 h in air. The average particle size, morphology, and particle size distribution were determined by a scanning electron microscope (SEM), and the transition temperature was measured by a four-probe method for the YBa2Cu3O7-x powders prepared at 920 °C. To take the SEM images of the powders, the fired compact samples were ground in a mortar and dissolved in a solvent with an ultrasonic treatment, followed by the coating of the dissolved samples on the carbon paper.
Sol formation was possible in a limited molar ratio of PVA to the total metal ions in the solution. Table 1 summarizes the appearance of the materials derived from the solutions at the various molar ratios of PVA to the total metal ions as a result of the evaporation of water. It was seen that a blue transparent sol formed from the solution when the molar ratio of PVA to the total metal ions was equal to or above 10. Transparency of the sol indicates that the composition of the sol was very homogeneous. During the evaporation of water, a white precipitate formed as the solution became concentrated in the range of the molar ratio of PVA to the total metal ions less than 10. It has been confirmed by X-ray diffraction that this white precipitate was barium nitrate (Chu and Dunn, 1987). Concerning the possible mechanism for the sol formation, Saha et al. (1995) suggested in their preparation of fine particles of the mixed oxide system using PVA that PVA helps the homogeneous distribution of the metal ions in its polymeric network structure and inhibits their segregation or precipitation from the solution. Li et al. (1993) also proposed in more detail the effect of poly(ethylene glycol) on the sol formation. From their proposal, therefore, it is speculated in our PVA-assisted sol-gel method that the hydroxyl polar ligands on the long chains of PVA can adsorb the metal ions in the solution and partially prevent them from meeting with each other. When the optimal amount of PVA is added, namely, when the molar ratio of PVA to metal ions is 10 in our study, PVA plays a role of wrapping and covering the metal ions to avoid the contact between them. Therefore, the metal ions will not grow in size and will not be precipitated, resulting in the formation of a cocoon-like structure in the PVA’s polymeric structure. If the amount of PVA added is too large, the metal ions may be covered by several layers of the polymer and the dehydration process becomes difficult because of the wrapping of water by PVA, although transparent sols are still forming. A schematic diagram of such a sol formation mechanism is represented in Figure 2. This cocoon-like structure could inhibit precipitation of the metal ions from the solution. On the other hand, it was seen that the molecular weight of PVA affected precipitation of the metal ions. The molecular weight of PVA used in the study was 1500. When the same molar ratio of PVA to the total metal ions with a molecular weight of 800 was used, precipitation could not be avoided even if the molar ratio exceeded 10. This was because when the molecular chains of the polymer were too short, the metal ions could not be completely covered by the polymer. As the molar ratio of PVA to the total metal ions increased, pH values of the initial solution were found to increase from the acidic condition to the neutral one. Although the pH of the solution affects the cation segregation in precipitation, hydrolysis, condensation, and conven-
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Figure 2. Proposed cocoon-like local structure formed by the interaction of hydroxide side groups on PVA with the metal ions during the sol formation.
Figure 5. Differential thermal analysis of the powdery precursor pretreated at 220 °C prior to DTA measurement.
Figure 3. Thermogravimetric analysis of (a) sole PVA and (b) the gel precursor at an air flowrate of 40 cm3/min and a heating rate of 5 °C/min.
Figure 4. Differential thermal analysis of (a) sole PVA and (b) the gel precursor.
tional sol-gel methods, such an effect was not as important as the amount of PVA in our preparation method because of the role of PVA as a protecting agent of the metal ions. Figures 3 and 4 show the results of TG and DTA for sole PVA and the gel precursor, respectively. According to the thermogravimetry as shown in Figure 3, the weight loss of the sole PVA continued up to 600 °C. On the other hand, the weight loss of the gel precursor terminated around 440 °C, and three discrete weight loss regions occurred at 40-140, 140-360, and 360440 °C, respectively. The weight loss in the tempera-
ture range of 40-140 °C corresponds to the removal of water, which is accompanied by an endothermic peak at 85 °C in Figure 4. The weight loss in the temperature range of 140-360 °C indicates the decomposition of the nitrates which occurs with an exothermic peak at 310 °C in Figure 4. The decomposition of the nitrates is an exothermic reaction, and, therefore, the nitrate ions act as an oxidizer, helping the decomposition of PVA (Saha et al., 1995). After this stage, it was found that the gel precursor turned into fluffy dark brown powders. The produced gases are presumed to be NO2 and CO2, if we consider the same kind of gel precursor formation by the citrate process (Baythoun and Sale, 1982). The weight loss in the temperature range of 360-440 °C corresponds to the decomposition of organic constituents. As indicated by Figure 4, the pyrolysis at this stage was very complicated. It is presumed that three exothermic peaks at 363, 392, and 425 °C in Figure 4b correspond to the heat of reaction for barium, copper, and yttrium ions remaining in the amorphous matrix to form barium carbonate, copper oxide, and yttrium oxide, respectively, with remaining PVA. This result is similar to the decomposition of the polymeric gel precursor by the amorphous citrate process into barium nitrate and barium carbonate at 300 °C and barium carbonate, copper oxide, and yttrium oxide at 500 °C (Chu and Dunn, 1987). Above 440 °C, the weight loss of the gel precursor was not observed. Compared to the extent of the weight loss, the magnitude of exothermic peaks in the temperature range of 360-440 °C for the gel precursor and those in the temperature range of 420-545 °C for sole PVA are quite big, which indicates the organic residues have been decomposed into oxidized carbons. In order to investigate a crystallization temperature of the YBa2Cu3O7-x superconducting phase, the DTA curve of the powdery (fluffy) precursor pretreated at 220 °C prior to DTA measurement is also represented in Figure 5. This result shows that the endothermic peak at about 800 °C corresponds to the crystallization of the YBa2Cu3O7-x superconducting phase. This crystallization temperature was 150 °C lower than the one for the solid-state reaction. Accordingly, it can be presumed that the homogeneously distributed metal ions catalytically enhanced the decomposition of the organic residues in the temperature range of 360-440 °C, lowering the complete decomposition temperature of PVA from 600 to 440 °C.
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Figure 7. Scanning electron micrograph of the material calcined at 920 °C for 2 h.
Figure 6. X-ray diffraction patterns of the gel and gel-derived materials heat-treated at various temperatures: (a) gel, (b) 500 °C, (c) 800 °C, (d) 900 °C, and (e) 920 °C. The meaning of the letters and symbols in the figure is as follows: B, BaCO3; C, CuO; Y, Y2O3; O, Y2Cu2O5; b, YBa2Cu3O7-x.
Figure 6 shows the X-ray diffraction patterns of the gel and gel-derived materials heated at various temperatures. The gel was found to be amorphous from the X-ray diffraction pattern. For the sample heated at 500 °C for 2 h, BaCO3, CuO, and Y2O3 were observed. When a material was calcined at 800 °C for 2 h, a significant amount of YBa2Cu3O7-x and a small amount of BaCO3 and Y2O3 were detected. This is quite consistent with the result of DTA which shows the crystallization peak at about 800 °C. Most of the intermediate products disappeared, and a single phase of YBa2Cu3O7-x was formed when the material was calcined at 900 °C for 2 h. As the material was calcined at 920 °C for 2 h, there was a gradual increase in the peak intensities accompanied by sharpening of the peaks at 2θ ) 46.6° and 58.1°. These results suggested that the formation of the YBa2Cu3O7-x superconducting phase was found to take place through the reaction among Y2O3, BaCO3, and CuO (Chu and Dunn, 1987). This reaction process is similar to the solid-state reaction where metal oxides or carbonates are utilized to form the superconducting phase at 950 °C for more than 24 h through several cooling, grinding, and heating steps, but the use of PVA greatly suppresses the formation of precipitates from which the heterogeneity stems. Thereby, the fine mixture state of calcined materials in the homogeneous composition makes it possible to form a single phase of YBa2Cu3O7-x under the mild condition. This may be ascribed to the fact that the materials derived from the sol are of atomic scale and homogeneously mixed with each other and thus have high sinterability. From the above results, our preparation method has advantages of lower calcination temperature and shorter calcination time over the solid-state reaction method. Compared with other solution methods (Brylewski and Przybylski, 1993; Johnson et al., 1987; Manthiram and Goodenough, 1987; Sanjines et al., 1988), the preparation conditions
of our method are comparable or even milder. Concerning the calcination temperature, most temperatures used are around 825-950 °C and the lowest reported so far is 780 °C, with the calcination time of 120 h and with the production of traces (70% of the particles, a median size between 21 and 23 nm, but the particle size seemed to be widely distributed. The particle size of oxalate-based YBa2Cu3O7-x powders was also reported to be as big as 330 nm in their study. Figure 8 shows the temperature dependence of dc electrical resistance for the pellet which was fabricated
4300 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996
Figure 8. Temperature dependence of resistivity for the material calcined at 920 °C for 2 h.
from the powders calcined at 920 °C for 2 h. The resistivity of the sample decreases gradually with decreasing temperature. The onset and zero temperature were 97.3 and 94 K, respectively. As can be seen in Figure 8, the transition range is narrow and equals ∆Tc ) 3.3 K, indicating that the material shows a very good metallic behavior. These observations reveal that an optimal intermetallic contact between YBa2Cu3O7-x was favored, since the obtained powders were composed of ultrafine monodispersed particles as shown in Figure 7. The gel precursors prepared in our study can be used as precursors for the development of superconductive wires in the future. Conclusion The high-Tc YBa2Cu3O7-x superconductor powders were prepared by the PVA-assisted sol-gel method, of which sol formation mechanism was essentially different from the conventional amorphous citrate method or Pechini method, using PVA as a protecting agent of metal ions. Transparent sols were obtained from the aqueous solution of metal nitrate containing PVA when the molar ratio of PVA to the total metal ions was equal to or above 10. As the evaporation of water proceeded, the sol turned into the viscous blue gel precursor, which was then followed by precalcination at 500 °C for 2 h in air to decompose and eliminate organic contents. Decomposition of PVA in the gel precursor terminated below 440 °C, which was 160 °C lower than the decomposition temperature for sole PVA, thus enabling crystallization of YBa2Cu3O7-x at 800 °C. A nearly pure phase of polycrystalline YBa2Cu3O7-x could be obtained by calcining the gel precursor above 900 °C for 2 h in air. It was seen that polycrystalline powders prepared at 920 °C consisted of very uniformly-sized ultrafine particulates with an average particle size of 25 nm and showed a good superconducting property with Tc around 94 K. Literature Cited Barboux, P.; Tarascon, J. M.; Greene, L. H.; Hull, G. W.; Bagley, B. G. Bulk and Thick Films of the Superconducting Phase YBa2Cu3O7-y Made by Controlled Precipitation and Sol-Gel Processes. J. Appl. Phys. 1988, 63 (8), 2725. Baythoun, M. S. G.; Sale, F. R. Production of StrontiumSubstituted Lanthanum Manganite Perovskite Powder by the Amorphous Citrate Process. J. Mater. Sci. 1982, 17, 2757.
Brylewski, T.; Przybylski, K. Physicochemical Properties of HighTc (Bi, Pb)-Sr-Ca-Cu-O Superconductors Prepared by SolGel Technique. Appl. Supercond. 1993, 1 (3-6), 737. Carry, C.; Mocellin, A. Superplastic Forming of Alumina. Br. J. Ceram. Soc. 1983, 33, 101. Chen, I.-W.; Keating, C. Y.; Wu, J.; Xu, J.; Reyes-Morel, P. E.; Tien, T. Y. Superconductivity and Tailoring of Lattice Parameters of the Compound YBa2Cu3Ox. Adv. Ceram. Mater. 1987, 2 (3B), 457. Chu, C. T.; Dunn, B. Preparation of High-Tc Superconducting Oxides by the Amorphous Citrate Process. J. Am. Ceram. Soc. 1987, 70 (12), C-375. Fransaer, J.; Roos, J. R.; Delaey, L.; van der Biest, O.; Arkens, O.; Celis, J. P. Sol-Gel Preparation of High-Tc Bi-Ca-SrCu-O and Y-Ba-Ca-O Superconductors. J. Appl. Phys. 1989, 65 (8), 3277. Johnson, S. M.; Gusman, M. I.; Rowcliffe, D. J.; Geballe, T. H.; Sun, J. Z. Preparation of Superconducting Powders by FreezeDrying. Adv. Ceram. Mater. 1987, 2 (3B), 337. Lessing, P. A. Mixed-Cation Oxide Powders via Polymeric Precursors. Ceram. Bull. 1989, 68 (5), 1002. Li, X.; Zhang, H.; Chi, F.; Li, S.; Xu, B.; Zhao, M. Synthesis of Nanocrystalline Composite Oxides La1-xSrxFe1-yCoyO3 with the Perovskite Structure Using Polyethylene Glycol-Gel Method. Mater. Sci. Eng. 1993, B18, 209. Manthiram, A.; Goodenough, J. B. Synthesis of the High-Tc Superconductor YBa2Cu3O7-δ in Small Particle Size. Nature (London) 1987, 329 (22), 701. Nozue, A.; Nasu, H.; Kamiya, K.; Tanaka, K. Transport Characteristics Related with Microstructure of (Bi, Pb)-Sr-Ca-Cu-O Superconductor Prepared by the Sol-Gel Method. J. Mater. Sci. 1991, 26, 4427. Pechini, M. Method of Preparing Lead and Alkaline-Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor. U.S. Patent 3,330,697, 1967. Saha, S. K.; Pathak, A.; Pramanik, P. Low-temperature Preparation of Fine Particles of Mixed Oxide Systems. J. Mater. Sci. Lett. 1995, 14, 35. Sanjines, R.; Thampi, K. R.; Kiwi, J. Preparation of Monodispersed Y-Ba-Cu-O Superconductor Particle via Sol-Gel Methods. J. Am. Ceram. Soc. 1988, 71 (12), C-512. Sun, Y. K.; Lee, W. H. Preparation of High Purity 110K Phase in the Bi(Pb)-Sr-Ca-Cu-O Superconductor using the Modified Citrate Process. Phys. C 1993, 212, 37. Venkatochasi, K. R.; Rai, R. Superplastic Flow in Fine-Grained Alumina. J. Am. Ceram. Soc. 1986, 69 (2), 135. Villa, P. L.; Zannella, S.; Ottoboni, V.; Ricca, A.; Ripamonti, N.; Scagliotti, M. YBa2Cu3Ox Preparation by the Amorphous Citrate Method. J. Less-Common Met. 1989, 150, 299. Wang, H. H.; Carlson, K. D.; Geiser, U.; Thorn, R. J.; Kao, H. C. I.; Beno, M. A.; Monaghan, M. R.; Allen, T. J.; Proksch, R. B.; Stupka, D. L.; Williams, J. M.; Flandermeyer, B. K.; Poeppel, R. B. Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-Tc Metal Oxide Superconductors La2-xSrxCuO4. Inorg. Chem. 1991, 26 (10), 1474
Received for review August 23, 1995 Revised manuscript received May 21, 1996 Accepted July 2, 1996X IE950527Y
X Abstract published in Advance ACS Abstracts, September 15, 1996.
